Generally today's hearing aids use a directionality system for determination of directionality of sounds detected by microphones placed on the hearing aids. Normally the directionality is determined by utilising two microphones on each hearing aid, which microphones are separated by a short distance, approximately 1 cm. The registered sounds are converted by the microphones to a first and second electric signal, which are compared. The difference between the first and second electric signal is a function of the location of the sound source, hence, the difference is utilised for selecting an appropriate directionality program in the processor of the hearing aid.
For example, European patent no.: EP 1 174 003 discloses a programmable multi-mode, multi-microphone system for use with a hearing aid. The system allows the user to select between a wide variety of modes or programs such as omni-directional mode, two-microphone directional mode, single-microphone directional mode and a mixed microphone and tele-coil mode.
Further international patent application no.: WO 01/54451 discloses a directional microphone assembly comprising a front and a rear microphone for a hearing aid, and comprising a processor, which generates a directional microphone output signal on the basis of the sound received at the front and rear microphones.
In addition, U.S. Pat. No. 6,778,674 discloses a hearing assist device comprising a first microphone, a second microphone, and circuitry for outputting a processed signal in response to position of sound source.
Neither of the above patent documents, which hereby are incorporated in the below specification by reference, realise and/or solve the problem of the fact that the length of the wavelengths of the lower frequencies are long relative to the distance between two directionality microphones. Generally the distance between the two directionality microphones on a hearing aid is approximately 1 cm. In these circumstances, in particular, the low frequency signals (e.g. smaller than 1000 Hz such as 500 Hz) recorded at each of the directionality microphones are substantially identical, and since the directionality is determined on the basis of difference between the signals of the two directionality microphones, the calculated directionality is mostly based on the high frequency elements of sounds. This problem may obviously be solved by introducing a frequency dependent gain amplifying the low frequency difference signal; however, this generally introduces amplification of noise, which is undesirable. Hence establishing directionality of low frequency signal in the present state of the art is unsatisfactory.